Modern day electrical measurement and computing circuits are used in a variety of applications and are increasingly being applied to actual production situations to perform various control operations such as process control, machine tool control, data monitoring and a variety of other applications which require the measurement and computing circuitry to sample input signals generated in the production environment, perform computations and produce output signals to the production environment to control various aspects of the manufacturing or process operations.
Input data to be processed by such electrical circuitry in a manufacturing environment is often "noisy" in that it contains many extraneous signals in addition to the desired signal. Often the extraneous signals are magnitudes larger than the desired signals. For example, in order to control a manufacturing process, a computer may be required to measure the output signals developed by a number of thermocouples which measure temperature variables at various points in the process. The output signal developed across the thermocouple during the measurement operation may be in the order of one millivolt. However, thermocouples typically operate in environments associated with electrical heaters and other electrical power elements which may generate large electrical transients across the thermocouple or place a high voltage on the thermocouple due to short circuits and other malfunctions. Thus the electrical potential between the thermocouple and the computer may be many hundreds of volts.
In order to detect low level input signals, sensitive circuits must be designed into the measurement circuitry. Such sensitive input circuitry would be destroyed if a large transient or extraneous high voltage signal were accidently applied to the circuitry and thus a direct connection between the computer and the sensing device is not possible.